This invention relates to underwater structures or devices for preventing erosion of river or sea beds. In one aspect, the invention relates to mats having a large number of buoyant strands (fronds) deployed to prevent erosion around underwater structures such as pilings, offshore structures, cables, and the like. In another aspect, the invention relates to structures for enhancing the accumulation of sediment on the sea floor.
Worldwide, the forces of nature are ceaselessly at work reshaping beaches, riverbeds, and the sea floor. Currents, tides, and wave action work in concert and individually to continually shift sediment. When structures such as pipelines and cables are placed in these environments, water is forced around these objects, causing local current velocities to increase. Higher current velocities, in turn, scour away the supporting sediment, resulting in erosion, destabilization, and eventual structure failure.
Historically, solutions for scour have included dumping rocks, placing concrete mattresses, or employing other remedial measures. In addition to causing damage and further scour on their own, these xe2x80x9csolutionsxe2x80x9d are extremely expensive and often short-lived.
More recent solutions to the erosion problem have been frond lines or mats. These devices employ a large number of buoyant synthetic strands (fronds) which, when deployed on the sea bed, resemble natural sea grass or seaweed. These artificial reed-like strands create sufficient viscous drag on currents flowing therethrough to cause sediments to deposit and accumulate in the region of deployment. The accumulation forms a permanent consolidated sandbank.
Representative examples of prior frond lines and mats are disclosed in U.S. Pat. Nos. 3,299,640; 4,337,007; 4,722,639; and 5,176,469. Many of these patents, particularly the more recent ones, disclose the preferred frond material to be polypropylene. While underwater erosion control mats made of polypropylene have performed well, and in fact are still performing well in their applications, they also have a number of shortcomings. These shortcomings have become evident in recent years, when the field of use of these mats was broadened beyond their initial applications. The first shortcoming is die-cuttability of the polypropylene film from which the strands, or fronds, are made, which is essential to achieve acceptable manufacturing costs. The second shortcoming is strand density, which needs to be as low as possible in order for these mats to perform well in fresh water and in shallow estuaries subject to tides. A third shortcoming is also associated with applications in shallow tidal areas, where part or all of the strands may be above water from time to time, and under these conditions are subject to ultraviolet radiation from the sun. A fourth shortcoming is that under certain circumstances, the strands, or fronds, of these mats can become covered with algae and the like, which increase their weight and cause the strands, or fronds, to lose their buoyancy. The invention, described in detail below, overcomes these four principal shortcomings, and gives underwater erosion control mats long-lasting utility in every imaginable salt and fresh water environment.
The erosion control mat of the present invention comprises a series of frond lines wherein each frond line includes an elongate base strip and a plurality of buoyant strands extending upwardly from the base strip. The strands are made of linear low density polyethylene (LLDPE), preferably a LLDPE produced by a metallocene or other single site catalysed process.
The base strip and strands are preferably integral, being formed from a single film sheet die in the form of a continuous base strip and a plurality of strands extending upwardly therefrom. The base strip may be folded longitudinally to provide a double layer for reinforcement.
The series of frond lines are preferably made from a continuous slitted film folded back and forth to form a fanfold array. This structure is deployed and anchored to the sea floor and provides a series of frond curtains through which water current flows. The viscous drag of water current flowing through these curtains of fronds causes the sediments to deposit and accumulate within this structure.